Fixes, thanks Gradle.

src/main/java/net/fabricmc/loom/api/remapping/RemapperExtension.java

@@ -26,28 +26,21 @@
 
 import javax.inject.Inject;
 
-import org.jetbrains.annotations.ApiStatus;
+import org.gradle.api.Action;
 import org.objectweb.asm.ClassVisitor;
 
 /**
  * A remapper extension can be used to add extra processing to the remapping process.
  *
- * <p>Implementations of RemapperExtension's are subject to the following constraints:
- * <ul>
- *     <li>Do not implement {@link #getParameters()} in your class, the method will be implemented by Gradle.</li>
- *     <li>Constructors must be annotated with {@link Inject}.</li>
- * </ul>
+ * <p>Implementations of RemapperExtension's must have the following:
+ * A single constructor annotated with {@link Inject}, and taking a single argument of the parameters.
+ * Or a single constructor annotated with {@link Inject} taking no arguments, when the extension does not have any parameters.
+ *
+ * <p>Use {@link net.fabricmc.loom.api.LoomGradleExtensionAPI#addRemapperExtension(Class, Class, Action)} to register a remapper extension.
  *
  * @param <T> Parameter type for the extension. Should be {@link RemapperParameters.None} if the action does not have parameters.
  */
 public interface RemapperExtension<T extends RemapperParameters> {
-	/**
-	 * @return The parameters for this extension.
-	 */
-	@ApiStatus.NonExtendable
-	@Inject
-	T getParameters();
-
 	/**
 	 * Return a {@link ClassVisitor} that will be used when remapping the given class.
 	 *

src/main/java/net/fabricmc/loom/api/remapping/RemapperParameters.java

@@ -24,13 +24,18 @@
 
 package net.fabricmc.loom.api.remapping;
 
+import org.jetbrains.annotations.ApiStatus;
+
 /**
  * Marker interface for parameter objects to {@link RemapperExtension}s.
  *
  * <p>Design based off of Gradle's {@link org.gradle.workers.WorkParameters}.
  */
 public interface RemapperParameters {
 	final class None implements RemapperParameters {
+		@ApiStatus.Internal
+		public static None INSTANCE = new None();
+
 		private None() {
 		}
 	}

src/main/java/net/fabricmc/loom/extension/LoomGradleExtensionApiImpl.java

@@ -34,6 +34,7 @@
 import org.gradle.api.artifacts.Dependency;
 import org.gradle.api.file.ConfigurableFileCollection;
 import org.gradle.api.file.RegularFileProperty;
+import org.gradle.api.model.ObjectFactory;
 import org.gradle.api.provider.ListProperty;
 import org.gradle.api.provider.Property;
 import org.gradle.api.provider.SetProperty;
@@ -390,10 +391,17 @@ public void createRemapConfigurations(SourceSet sourceSet) {
 
 	@Override
 	public <T extends RemapperParameters> void addRemapperExtension(Class<RemapperExtension<T>> remapperExtensionClass, Class<T> parametersClass, Action<T> parameterAction) {
-		T parameters = getProject().getObjects().newInstance(parametersClass);
-		parameterAction.execute(parameters);
+		final ObjectFactory objectFactory = getProject().getObjects();
+		final RemapperExtensionHolder holder;
+
+		if (parametersClass != RemapperParameters.None.class) {
+			T parameters = objectFactory.newInstance(parametersClass);
+			parameterAction.execute(parameters);
+			holder = objectFactory.newInstance(RemapperExtensionHolder.class, parameters);
+		} else {
+			holder = objectFactory.newInstance(RemapperExtensionHolder.class, RemapperParameters.None.INSTANCE);
+		}
 
-		RemapperExtensionHolder holder = getProject().getObjects().newInstance(RemapperExtensionHolder.class, parameters);
 		holder.getRemapperExtensionClassName().set(remapperExtensionClass.getName());
 		remapperExtensions.add(holder);
 	}

src/main/java/net/fabricmc/loom/extension/RemapperExtensionHolder.java

@@ -30,6 +30,7 @@
 import org.gradle.api.provider.Property;
 import org.gradle.api.tasks.Input;
 import org.gradle.api.tasks.Nested;
+import org.gradle.api.tasks.Optional;
 import org.jetbrains.annotations.Nullable;
 import org.objectweb.asm.ClassVisitor;
 import org.objectweb.asm.commons.Remapper;
@@ -42,6 +43,7 @@
 import net.fabricmc.tinyremapper.api.TrClass;
 
 public abstract class RemapperExtensionHolder {
+	// Null when RemapperParameters.None.class
 	private final RemapperParameters remapperParameters;
 
 	@Inject
@@ -53,6 +55,7 @@ public RemapperExtensionHolder(RemapperParameters remapperParameters) {
 	public abstract Property<String> getRemapperExtensionClassName();
 
 	@Nested
+	@Optional
 	public RemapperParameters getRemapperParameters() {
 		return remapperParameters;
 	}
@@ -81,7 +84,12 @@ private RemapperExtension<?> newInstance(ObjectFactory objectFactory) {
 		try {
 			Class<? extends RemapperExtension> remapperExtensionClass = Class.forName(getRemapperExtensionClassName().get())
 					.asSubclass(RemapperExtension.class);
-			return objectFactory.newInstance(remapperExtensionClass, getRemapperParameters());
+
+			if (remapperParameters == RemapperParameters.None.INSTANCE) {
+				return objectFactory.newInstance(remapperExtensionClass);
+			}
+
+			return objectFactory.newInstance(remapperExtensionClass, remapperParameters);
 		} catch (Exception e) {
 			throw new RuntimeException("Failed to create remapper extension", e);
 		}

src/test/groovy/net/fabricmc/loom/test/integration/buildSrc/remapext/TestPlugin.groovy

@@ -28,6 +28,7 @@ import org.gradle.api.Plugin
 import org.gradle.api.Project
 
 import net.fabricmc.loom.LoomGradleExtension
+import net.fabricmc.loom.api.remapping.RemapperParameters
 
 class TestPlugin implements Plugin<Project> {
 	@Override
@@ -36,5 +37,7 @@ class TestPlugin implements Plugin<Project> {
 		extension.addRemapperExtension(TestRemapperExtension.class, TestRemapperExtension.Params.class) { TestRemapperExtension.Params p ->
 			p.replacements.put("Hello World!", "Hello Loom!")
 		}
+		extension.addRemapperExtension(TestTinyRemapperExtension.class, RemapperParameters.None.class) { RemapperParameters.None p ->
+		}
 	}
 }

src/test/groovy/net/fabricmc/loom/test/integration/buildSrc/remapext/TestRemapperExtension.groovy

@@ -24,6 +24,8 @@
 
 package net.fabricmc.loom.test.integration.buildSrc.remapext
 
+import javax.inject.Inject
+
 import org.gradle.api.provider.MapProperty
 import org.objectweb.asm.ClassVisitor
 
@@ -33,9 +35,16 @@ import net.fabricmc.loom.api.remapping.RemapperParameters
 import net.fabricmc.loom.util.Constants
 
 abstract class TestRemapperExtension implements RemapperExtension<Params> {
+	final Params parameters
+
+	@Inject
+	TestRemapperExtension(Params parameters) {
+		this.parameters = parameters
+	}
+
 	@Override
 	ClassVisitor insertVisitor(String className, RemapperContext remapperContext, ClassVisitor classVisitor) {
-		def replacements = getParameters().replacements.get()
+		def replacements = parameters.replacements.get()
 		return new StringReplacementClassVisitor(Constants.ASM_VERSION, classVisitor, replacements)
 	}
 

src/test/groovy/net/fabricmc/loom/test/integration/buildSrc/remapext/TestTinyRemapperExtension.groovy

@@ -0,0 +1,55 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2023 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom.test.integration.buildSrc.remapext
+
+import org.objectweb.asm.ClassVisitor
+
+import net.fabricmc.loom.api.remapping.RemapperContext
+import net.fabricmc.loom.api.remapping.RemapperExtension
+import net.fabricmc.loom.api.remapping.RemapperParameters
+import net.fabricmc.loom.api.remapping.TinyRemapperExtension
+import net.fabricmc.tinyremapper.TinyRemapper
+
+class TestTinyRemapperExtension implements RemapperExtension<RemapperParameters.None>, TinyRemapperExtension {
+	@Override
+	ClassVisitor insertVisitor(String className, RemapperContext remapperContext, ClassVisitor classVisitor) {
+		return classVisitor
+	}
+
+	@Override
+	TinyRemapper.AnalyzeVisitorProvider getAnalyzeVisitorProvider() {
+		return null
+	}
+
+	@Override
+	TinyRemapper.ApplyVisitorProvider getPreApplyVisitor() {
+		return null
+	}
+
+	@Override
+	TinyRemapper.ApplyVisitorProvider getPostApplyVisitor() {
+		return null
+	}
+}